The present invention refers to a new and improved electronic weft thread monitor on a gripper shuttle weaving machine provided with a picking device, a catch box with means for braking and returning, after weft thread insertion, the gripper shuttle to a defined thread releasing position, and a thread tensioner arranged between a fixed thread guide eye and the picking device and serving for deviating and tensioning the weft thread during the push back motion of the gripper shuttle, the weft thread monitor comprising a sensing device located in the range between the thread guide eye and the thread tensioner, and a switching device for producing a control signal defining the time interval in which the weft thread is monitored.
Weaving machines provided with gripper shuttles, also termed projectiles, are known and in worldwide use. By way of example, a loom of this type is described in Swiss Pat. No. 399,354 (German Pat. No. 1,535,615). There is also disclosed a weft thread monitoring device which responds to the so-called pull after motion of the weft thread occurring in the zone of the thread tensioner during the last phase of the tensioning process, i.e. when the weft or filling thread has been inserted in the weaving shed and is in a tensioned condition.
Further, in Swiss Pat. No. 489,642 there is described an electronic weft thread monitoring device on a gripper shuttle weaving machine, comprising a thread sensing device or sensing head arranged in the zone of the thread tensioner. This device serves for monitoring the weft insertion into the weaving shed within a so-called control interval defined by particular control circuitry. It is imperative to confine the monitoring process to such a control interval since otherwise the weft sensing signal furnished by the sensing head and disappearing at the end of each weft insertion--even with intact weft thread--would simulate a weft break.
A triboelectrical sensing head suitable for a weft thread monitor of the present type is described, by way of example, in Swiss Pat. No. 479,478 or the corresponding U.S. Pat. No. 3,676,769.
For the sake of a more reliable indication of weft breaks and other weft faults it is advantageous to survey the weft insertion as well as the following weft tensioning phase. A weft yarn or filling thread monitor of this type is described in U.S. Pat. No. 4,228,828. Here, two separate successive sensing signals occur, one for the weft insertion and another one for the tensioning phase. Accordingly, two successive control intervals or signals are provided for the said separate sensing signals. The precise setting of these control signals, however, is rather difficult in practice because of the unavoidable fluctuations of the duration of the weft insertion and tensioning phases.
Another problem arises when the weft insertion in multicolour looms of the type in question is to be monitored. Usually, an individual sensing device is provided for each of the weft threads, and all these sensing devices are connected to a common signal circuit. Here the idle threads which are not to be inserted into the weaving shed are in contact with their individual sensing heads, so that these threads when vibrated by the operating loom may produce spurious signals. Now when the weft thread which is being inserted into the shed breaks, those spurious signals may simulate a thread travel and thus supersede the stop mechanism of the loom.